I rode a little at my bike shop this weekend. The highest end components I tried were SLX RD on a 2014 Trek DS 8.5. I tried to exercise both derailleurs as much as possible and I didn't notice much difference at all between that and my 20 year old components. In fact, they were quite a bit louder too. Maybe not lubricated yet.
I understand my ability to really put them thru their paces is limited on a test ride. But I didn't see the seamless shifting or value to drop over $1K. And believe me, I was looking for every excuse to get a new bike.
I liked the front suspension but, after all my research, was surprised at the lack of difference in shifting between a new SLX and my old 300LX.